VCA2611 데이터 시트보기 (PDF) - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS

부품명

상세내역

제조사

VCA2611

Dual, Variable-Gain Amplifier with Low-Noise Preamp

TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS 

PGA OVERVIEW AND OVERALL DEVICE

CHARACTERISTICS

The differential output of the VCA attenuator is then amplified

by the PGA circuit block. This post-amplifier is programmed

by the same MGS bits that control the VCA attenuator,

yielding an overall swept-gain amplifier characteristic in which

the VCA × PGA gain varies from 0dB (unity) to a program-

mable peak gain of 24-, 27-, 30-, 33-, 36-, 39-, 42-, or 45dB.

The Gain vs VCACNTL curve in the Typical Characteristics

shows the composite gain control characteristic of the entire

VCA2616. Setting VCACNTL to 3.0V causes the digital MGS

gain control to step in 3dB increments. Setting VCACNTL to 0V

causes all the MGS-controlled gain curves to converge at

one point. The gain at the convergence point is the LNP gain

less 6dB, because the measurement setup looks at only one

side of the differential PGA output, resulting in 6dB lower

signal amplitude.

ADDITIONAL FEATURES—OVERVIEW

Overload protection stages are placed between the attenua-

tor and the PGA, providing a symmetrically clipped output

whenever the input becomes large enough to overload the

PGA. A comparator senses the overload signal amplitude

and substitutes a fixed DC level to prevent undesirable

overload recovery effects. As with the previous stages, the

VCA is AC-coupled into the PGA. In this case, the coupling

time constant varies from 5µs at the highest gain (45dB) to

59µs at the lowest gain (25dB).

The VCA2616 includes a built-in reference, common to both

channels, to supply a regulated voltage for critical areas of

the circuit. This reduces the susceptibility to power-supply

variation, ripple, and noise. In addition, separate power

supply and ground connections are provided for each chan-

nel and for the reference circuitry, further reducing interchannel

crosstalk.

Further details regarding the design, operation, and use of

each circuit block are provided in the following sections.

LOW-NOISE PREAMPLIFIER (LNP)—DETAIL

The LNP is designed to achieve a low-noise figure, espe-

cially when employing active termination. Figure 4 is a

simplified schematic of the LNP, illustrating the differential

input and output capability. The input stage employs low

resistance local feedback to achieve stable low-noise, low-

distortion performance with very high input impedance. Nor-

mally, low noise circuits exhibit high power consumption as

a result of the large bias currents required in both input and

output stages. The LNP uses a patented technique that

combines the input and output stages such that they share

the same bias current. Transistors Q4 and Q5 amplify the

signal at the gate-source input of Q4, the +IN side of the LNP.

The signal is further amplified by the Q1 and Q2 stage, and

then by the final Q3 and RL gain stage, which uses the same

bias current as the input devices Q4 and Q5. Devices Q6

through Q10 play the same role for signals on the –IN side.

The differential gain of the LNP is given in Equation 1:

Gain

=

2

×







RL

RS







(1)

COMP2A

VDD

CCOMP

(External

Capacitor)

Q2

Q3

RW

Q4

LNPINP

Q1

RL

93Ω

LNPOUTN LNPOUTP

Buffer

Buffer

RS1

105Ω

RS2

34Ω

RS3

17Ω

LNPGS2

LNPGS1

LNPGS3

Q5

COMP1A

RL

93Ω

Q9

Q8

To Bias

Circuitry

Q7

RW

LNPINN

Q10

To Bias

Circuitry

Q6

FIGURE 4. Schematic of the Low-Noise Preamplifier (LNP).

10

www.ti.com

VCA2616, VCA2611

SBOS234D

Share Link: